The present invention relates to an ink jet printer having a bubble purge mechanism.
A print head of a conventional ink jet printer is shown in FIG. 7. The print head includes a nozzle plate 51, and a main body 52. The nozzle plate 51 is formed with a nozzle 51a from which ink is ejected. The main body 52 is formed with a chamber 52a in fluid connection with the nozzle 51a. A manifold 55 is defined in the main body 52 for supplying ink to the chamber 52a. The manifold 55 is in fluid connection with an ink tank (not shown) storing ink. A diaphragm 53 fluidly connects the chamber 52a with the manifold 55.
A piezoelectric element 54 is disposed at a position to the rear of the diaphragm 53 and in opposition with the chamber 52a. The print head is connected to a control portion (not shown). The control portion supplies control signals for actuating the piezoelectric element 54, which deforms to protrude toward the chamber 52a. The deformation of the piezoelectric element 54 applies pressure to the diaphragm 53, thereby transmitting the pressure to the chamber 52a. The increase in pressure in the chamber 52a ejects ink from the chamber 52a through the nozzle 51a as an ink droplet.
There has been known a problem in that bubbles mixed with the ink can absorb some of the pressure applied by the piezoelectric element 54 to the chamber 52a. The remaining pressure may be insufficient for ejecting an ink droplet. This can result in poor quality of printed characters and images. It is therefore desirable that bubbles be prevented from entering the chamber 52a.
For purging bubbles, a check valve mechanism including a ball 57 and a spring 58 are provided. A discharge passage 59 having a valve port 59a and a discharge port 59b is formed in the main body 52. The discharge passage 59 is in selective fluid communication with the manifold 55 by the action of the check valve mechanism. To purge bubbles, the ink tank (not shown) is subjected to a controlled pressure so that the a controlled ink pressure can be applied into the manifold 55. If the applied ink pressure is insufficient, the mixture of ink and bubbles in the manifold 55 is discharged outside through the nozzle 51a. On the other hand, if sufficient ink pressure is applied, the mixture of ink and bubbles is ejected through the nozzle 51a as well as through the discharge port 59b because the ink pressure overcomes the biasing force of the spring 58 so that the ball 57 is moved away from the valve port 59a.
In summary, the discharge port 59b can enhance bubble discharging ability, because the bubbles may not be sufficiently discharged through the nozzles 51a due to their small cross-sectional area. If high ink pressure is applied, most of the bubbles can be discharged through the discharge port 59b. Then, bubbles remaining in the nozzles 51a can be discharged outside through the nozzles 51a by applying low ink pressure.
There is a problem with this mechanism in that a great deal of ink must be ejected to purge bubbles. This wastes ink. To reduce the amount of wasted ink, several methods have been proposed which involve selectively switching between high and low pressure and varying the duration and number of pressurization. Still however, the conventional methods always discharge ink outside when purging. Therefore, ink wasting problem still remains unsolved.